The present invention is directed to a method of determining the degree of saturation of an adsorbent material. More particularly, it is directed to measuring changes in conductivity to establish a change in adsorptive capacity.
In many industrial processes the moisture content of gaseous streams should be reduced to a very low level, to prevent corrosion (production of dodecylbenzylsulfonic acid used in detergent industry) and/or side reactions (hydrolysis of reactants). Fixed bed dryers are generally used where the moisture content of the gases is reduced by adsorption/absorption on solid sorbents. Water molecules are entrapped in the capillaries on the particle surface. Depending on the type of the adsorbent, unwanted side products might form. In order to avoid the formation of such byproducts, some sorbents must be pretreated. After the pretreatment the sorbent is used on-stream until completely saturated. The saturated sorbent is usually regenerated by desorbing water to be used again for drying. The saturation-drying cycle is repeated many times. A regeneration factor can be defined which shows what percentage of the absorbed water can be removed and what capacity will be available for the next cycle.
%regeneration factor=The amount of desorbed moisture/Total amount of absorbed water (completely saturated)xc3x97100 
100% regeneration factor means complete removal of the absorbed water.
Other adsorbents such as activated charcoal are used to adsorb hydrocarbon fumes. The activity of the adsorbent gradually decreases as it becomes saturated by hydrocarbons. The spent adsorbent must then be replaced or regenerated. The present invention is directed to a method for determining the relative capacity/activity of activated charcoal based on conductivity measurements.
Conductivity measurements in the solid phase are relatively easy and are used to characterize materials containing ionic species. For instance, the electrical conductivity of pure and doped Fe2O3, and the effect of gamma-irradiation on the electrical conductivity was studied (M. A. Mousa, E. A. Gomaa, A. A. El-Khouly and A. A. M. Aly: Mater. Chem. Phys. 11, 433 (1984)). Doping either increased or decreased the electrical conductivity of pure Fe2O3("sgr"*=1.5xc3x9710xe2x88x926 [xcexa9xe2x88x921 cmxe2x88x921]), depending on the type and amount of doping elements. Gamma-irradiation increased the conductivity of pure Fe2O3, which in turn decreased upon annealing. Higher doses caused higher conductivity increases, which was explained by increasing charge carrier (Fe2+) concentrations. The above mentioned paper investigated these oxides in terms of semiconducting properties.
Soviet Union Patent No. 1221571 discusses monitoring of the decomposition of peroxides using a semiconductor sensor. Free electrons forming during the decomposition of the peroxides affect the electrical properties of the sensor. This differs fundamentally from the present invention in which ionic compounds are involved.
Japanese Patent No. HEI 1-253645 discloses a tube type of bed in which electrodes are placed at the entrance and exit of the tube. In this circuit, the bed represents a certain resistance which is greater than the resistance of the carbon due to voids between particles. When a solvent saturates the bed and fills the cavities, the resistance of the bed decreases. This is essentially the same as shorting out a circuit by moisture; the vapor/liquid provides an alternative conducting route. It is clearly apparent that this method is only useful for measuring gross changes in conductivity.
The present disclosure is directed to a method of measuring changes in conductivity as an indicator of relative adsorbent activity of a sorptive substance, such as an alumina sorbent or a decontaminating substance such as activated charcoal. Resistance is measured on a single bead or in a packed bed. As the degree of adsorption increases, the route of electrons is blocked and the resistance increases. The method of the current invention is capable of detecting changes in conductivity orders of magnitude smaller than the changes detectable by the Japanese method. The high level of sensitivity of the present invention ensures that the degree of saturation can be monitored to ensure that absolutely no break-through occurs. This is essential in industrial applications where the degree of saturation is rarely allowed to exceed 20%. Conductivity measurements are determined in the adsorptive phase. The adsorptive phase is defined as the stage at which all the sorbent is held to the sorptive material by physical or chemical means. Thus, for the present invention, in contrast to the above mentioned Japanese patent, as soon as molecules are in free phase, the adsorptive material is already saturated.
The present invention is directed to a method of monitoring the relative capacity of adsorbents at various stages of saturation by measuring an electrical property of the adsorbent and correlating a value for conductivity to adsorptive capacity. Changes in conductivity are determined during the adsorptive phase in which the material to be adsorbed is bound to the adsorptive material by physical or chemical means.